demandimport: blacklist sqlalchemy.events as it has side effects (issue5085)

Importing sqlalchemy.events cannot be delayed as it registers classes to
their event mechanism. It worked fine before 4f1144c3c72b, since they use
new-style imports. But now we have to blacklist it because our demandimport
can handle new-style imports.

This patch series isn't intended for stable as we don't guarantee API
compatibility with 3rd-party extensions. They can temporarily disable the
demand importer to work around the issue.

mercurial/demandimport.py

@@ -255,6 +255,7 b' ignore = ['
     '_sre', # issue4920
     'rfc822',
     'mimetools',
+    'sqlalchemy.events', # has import-time side effects (issue5085)
     # setuptools 8 expects this module to explode early when not on windows
     'distutils.msvc9compiler'
     ]